Solar energy is receiving wide-spread attention as a means of reducing the demand on conventional energy sources. Most activity has centered on using solar energy to provide heat and electricity; however, solar energy may also be used to drive energy-intensive chemical processes, such as the splitting of hydrogen from water.
The feasibility of using solar energy for various purposes in many cases depends on the practicality of energy storage systems to make the energy from the intermittent source continuously available. A plant using solar ray collectors as its source of energy and having an efficient energy storage system may be built to one-fifth or less the size of a plant with the same output capacity that does not have an energy storage system.
Means of storing energy to levelize the energy obtainable from intermittent sources include sensible heat reservoirs, such as rocks, oil or molten salts, heat-of-fusion or vaporization reservoirs, such as metals, compounds, salt compositions or hydrates and chemical heat storage cycles. The criteria for choosing a heat storage system include energy density, energy quality, energy delivery requirements and media stability, and different types of heat storage may be preferred for different processes. Sensible heat storage facilities require large volumes of heat-retentive material and are undesirable in this respect. Heat-of-fusion salt compositions have minimal energy delivering requirements, and sensible heat inserted into the salt compositions can be withdrawn with only heat exchanger .DELTA.T losses. The heat obtainable from molten salts approach theoretical efficiency. Hence, molten salts are often the energy storage systems of choice for short-term storage, e.g., to levelize energy during the diurnal cycle. For longer term storage, tankage and inventory costs of salt heat storage systems become critical. Furthermore, the maximum draw salt temperature in state-of-the-art salt heat storage systems is about 600.degree. C. limiting the usefulness of heat storage systems in processes which require high temperature heat. For long-term heat storage, chemical cycles can prove to be the most cost effective, and a system which depends upon generally uniform availability of solar energy over the seasonal cycle requires an efficient, economical chemical heat storage cycle. An important consideration in the evaluation of chemical heat storage systems is the cost of chemical tankage, and to this end, the inventory products of the heat storage systems should be liquids and/or solids rather than gases. Such chemical energy storage systems generally provide much higher energy densities than other energy storage systems requiring far less tankage volume than sensible or heat-of-fusion systems.
The need continues for efficient, economical heat storage systems, particularly those which can deliver high temperature heat.